This section of this document introduces various aspects of the art that may be related to various aspects of the present invention described and/or claimed below. It provides background information to facilitate a better understanding of the various aspects of the present invention. As the section's title implies, this is a discussion of “related” art. That such art is related in no way implies that it is also “prior” art. The related art may or may not be prior art. The discussion in this section of this document is to be read in this light, and not as admissions of prior art.
So-called super-thin frequency independent antennas, while having thin profiles suitable for missile surface conformal placement reducing volume demands, also have narrow bandwidth coverage for antenna types that are supposed to be “frequency independent” but are not.
Spirals antennas are known for much greater bandwidth, typically in practice 2-18 GHz. Spiral antennas are desired for their frequency independence in terms of impedance and antenna power pattern behavior (beamwidth, gain, etc.). Relative invariance in their antenna patterns over this region makes them attractive for passive reception of active sources over broad frequency ranges. Thin designs are dependent on and constricted by fixed circuit elements tuning their resonant RF cavity. It is this static lumped element technology that confines thin spiral bandwidths.
In the past deep cavities have been used on standard spiral antennas. Their cavities are set to a depth equal to one-quarter wavelength of their lowest frequency. Thus, at 2 GHz, a 1.5″ deep cavity is required far in excess of our volume allowance constrained by volume starved missiles. Some super thin cavities have also been employed with narrow bands (300 MHz compared to standard 16 GHz bands for typical spirals). Both are means of capturing and removing backwaves from the radiating element. While this latter means allows super thin spirals to operate (albeit over narrow band) thus providing their thin packaging advantage, it does not serve the broad frequency band desires expected of typical frequency independent spirals.
Whether cavities or lumped elements are employed, both act as a type of filter for their associated antenna, allowing undisturbed reception of frequencies within their bandpass while attenuating those frequencies outside their designed band. In this case the effect of out-of-band attenuation is to reduce antenna directivity. Out-of-band frequencies will cause the antenna backwave to destructively interfere with the forward propagating wave thereby reducing antenna mainbeam gain.
Most tunable RF filters fall into three basic types: mechanically tunable, magnetically tunable and electronically tunable filters. Mechanically tuned RF filters have large power handling capability, low insertion loss, tend to be large, heavy and switch at slow speeds. Magnetically tunable RF filters like YIG filters often used between 0.5-18 GHz are smaller, have moderate insertion loss (˜6 dB), moderate tuning speeds (2 GHz/ms) and require tuning currents of hundreds of milliamps. Electronically tunable RF filters are faster, smaller and adjust by variation in voltage, changing a capacitor's capacitance within the resonator (via varactor diodes, which are nonlinear and thus create intermod products; ferroelectric thin films, which are also nonlinear; or MEMS, switches/varactors). All have relatively narrow bandwidths, thus banks must be assembled to accommodate wide bands.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.